A polyethylene-based resin foam is generally used as a packing material owing to its high resilience and excellent oil and impact resistance. The polyethylene-based resin foam, however, has drawbacks that its stiffness and compressive strength are low. On the other hand, a polystyrene-based resin foam is excellent in stiffness, but has a drawback that it is brittle.
To overcome such drawbacks, Japanese Examined Patent Publication No. SHO 51 (1976)-46138, Japanese Examined Patent Publication No. SHO 52 (1977)-10150, Japanese Examined Patent Publication No. SHO 58 (1983)-53003, and Japanese Unexamined Patent Publication No. SHO 62 (1987)-59642 disclose methods for obtaining expandable particles of a styrene-modified polyethylene-based resin by impregnating a styrene monomer into a polyethylene-based resin for polymerization.
Examples of the polyethylene-based resin used in the method substantially include low-density polyethylene, high-density polyethylene, and an ethylene-vinyl acetate copolymer. However, where these resins are used, styrene is not sufficiently dispersed into polyethylene, and thus polyethylene needs to be cross-linked to provide sufficient stiffness and impact resistance. The cross-linking of polyethylene, however, causes a gel component comprising a cross-linked polymer to be generated.
To solve the above-mentioned problem, Japanese Patent No. 2668384 discloses a method for obtaining an expanded molded article of a modified polyethylene-based resin excellent in stiffness and impact resistance. In this method, 100 parts by weight of non-crosslinked linear low-density polyethylene-based resin particles, 5 to 300 parts by weight of a vinyl aromatic monomer, and 1 to 3 parts by weight of a polymerization initiator relative to 100 parts by weight of the vinyl aromatic monomer are dispersed in an aqueous medium. Then, a suspension thus obtained is heated at such a temperature that polymerization of the monomer does not substantially take place for impregnation of the monomer into an inside and a surface of the polyethylene-based resin particles. Subsequently, the temperature of the suspension is raised to polymerize the monomer, as a result the expanded molded article of the modified polyethylene-based resin is obtained by micro-dispersion of a vinyl aromatic polymer in polyethylene.
In this method, however, the polymerization is carried out after the monomer is fully impregnated into polyethylene. Accordingly, a monomer content in polyethylene is limited. When a large amount of the monomer needs to be impregnated into polyethylene, there is a problem that the remaining monomer which is unimpregnated into polyethylene is polymerized and that a large amount of polymer powder is generated. Also, where a ratio of polyethylene to the vinyl aromatic polymer is high, a retention of a blowing agent is poor and thus a reduction in density of the expanded molded article is difficult. Furthermore, since an amount of the polymerization initiator used in the method is as high as 1 to 3 parts by weight, a molecular weight of the vinyl aromatic polymer is small, making it difficult to provide a molded article having a sufficient strength.
In Examples of the above-mentioned patent, a styrene monomer is added to linear low-density polyethylene-based resin particles having a melting point of 122° C. for polymerization at 115° C. (the melting point of the resin particles is not specified in Examples, but the present inventors confirmed from the product name of the resin particles described in Examples that the particles have the above-mentioned melting point). The polymerization at this temperature often results in graft polymerization of styrene monomer on a polyethylene chain. Consequently, in a resin obtained, a gel component derived from cross-linking is not generated, but instead, a gel component derived from the graft polymerization is generated. Hereinafter, the term “graft polymer” means a gel component containing polystyrene, and the term “crosslinked polymer” means a gel component substantially not containing polystyrene.
Where styrene monomer is impregnated into a polyethylene-based resin for polymerization to obtain a resin which is subsequently impregnated with a blowing agent and molded by heating to obtain an expanded molded article, polyethylene needs to be cross-linked as mentioned above for providing sufficient stiffness and impact resistance. However, when the expanded molded product thus obtained is collected for recycled use thereof, a gel component generated by the cross-linking is contained in a molten resin of the expanded molded article, thereby causing the recycling of the expanded molded article to be difficult.
Accordingly, there has been desired development of expandable particles of a styrene-modified linear low-density polyethylene-based resin which are capable of widely changing a ratio of styrene monomer to polyethylene and suppressing generation of a gel component derived from cross-linking or graft polymerization, and which provide an expanded molded article having sufficient strength.